Ammonia refrigeration system

ABSTRACT

In accordance with the present disclosure there is provided an efficient refrigeration system for cooling an environment using ammonia and sodium thiocyanate using a low grade heat source. The refrigeration system comprises a condenser for condensing vaporous ammonia to liquid ammonia coupled to an evaporator. The condenser condenses the ammonia vapour using a heat exchanger. The evaporator evaporates liquid ammonia to vaporous ammonia by absorbing heat from the cooling environment. An absorber absorbs the vaporous ammonia into an ammonia sodium thiocyanate solution and adsorbs vaporous ammonia on to sodium thiocyanate salts. The absorber is coupled to a regenerator through a solution pump for pumping ammonia sodium thiocyanate solution with dissolved ammonia from the absorber to the regenerator. The regenerator regenerates ammonia vapour from the pumped solution and supplies the regenerated ammonia vapour to the condenser and the concentrated solution back to the absorber.

TECHNICAL FIELD

The present disclosure relates to refrigeration systems and more particularly to efficient ammonia refrigeration systems.

BACKGROUND

FIG. 1 depicts in a block diagram, a prior art refrigeration system 100. The system 100 is a generator, absorber, heat-exchanger (GAX) type refrigeration system. The system 100 comprises a generator 105, a heat exchanger 110, an absorber 115, an evaporator 120 and a condenser 125. The system uses a refrigerant-absorbent pair, for example, ammonia/water or water/LiBr.

The regenerator 105 uses heat, typically supplied from a gas fired heater, to regenerate the refrigerant from the refrigerant-absorbent solution. The heat causes the refrigerant to separate from the absorbent. The refrigerant is supplied to the condenser 125, which condenses the refrigerant vapour to liquid. The regenerator supplies the separated absorbent to the absorber 115, typically through an expander valve 117.

The condensed refrigerant is supplied to the evaporator 120, typically through an expander valve 122. The expansion of the refrigerant in the evaporator 120 causes the refrigerant to evaporate and absorb heat. The heat is absorbed from the environment to be refrigerated. The refrigerant vapour is sent to the absorber where it is absorbed by the absorbent. The absorption of the refrigerant vapour into the absorbent releases heat. This heat is dissipated into the environment, such as by air or water cooling. The refrigerant-absorbent solution is pumped to the regenerator 115. The solution passes through a heat exchanger 110, and absorbing heat from the absorbent from the regenerator.

Although the above system provides refrigeration, the system may require a large input of heat in order to regenerate the absorbent and refrigerant.

SUMMARY

In accordance with the present disclosure there is provided a refrigeration system for cooling a cooling environment using a low grade heat source. The refrigeration system comprises a condenser for condensing vaporous ammonia to liquid ammonia using a first heat exchanger, an evaporator coupled to the condenser through a first valve, an absorber coupled to the evaporator, a regenerator for regenerating ammonia vapour and a solution pump for pumping ammonia sodium thiocyanate solution with dissolved ammonia from the absorber to the regenerator. The evaporator evaporates liquid ammonia to vaporous ammonia by absorbing heat from the cooling environment, the evaporator receives the liquid ammonia from condenser. The absorber absorbs vaporous ammonia into an ammonia sodium thiocyanate solution and adsorbs vaporous ammonia onto sodium thiocyanate salts, the absorber receives the vaporous ammonia from the evaporator. The regenerator regenerates the ammonia vapour from the ammonia sodium thiocyanate solution with the absorbed vaporous ammonia, the regenerator is coupled to the condenser and supplies the regenerated ammonia vapour to the condenser. The regenerator is further coupled to the absorber for returning the concentrated thiocyanate solution to the absorber.

In accordance with the present disclosure there is further provided an absorber for use in a refrigeration system. The absorber comprises an outer shell comprising an outlet for removing ammonia sodium thiocyanate solution, an inlet for adding concentrated ammonia sodium thiocyanate solution to the absorber, and an inner cavity. The inner cavity holds ammonia sodium thiocyanate solution for absorbing ammonia vapour, and sodium thiocyanate salts for adsorbing ammonia vapour. The absorber further comprises an inner tube having an inlet and outlet end, the inner tube located within the inner cavity of the outer shell, and a gas duct for transporting ammonia vapour into the ammonia sodium thiocyanate solution within the inner tube from the inlet end to the outlet end.

In accordance with the present disclosure there is further provided a regenerator for regenerating ammonia vapour from a solution of ammonia sodium thiocyanate for use in a refrigeration system using a low grade heat source. The regenerator comprises an outer shell having an inner cavity and a circulation means for circulating a heat transfer medium from the inner cavity to a low grade heat exchanger for exchanging heat from the low grade heat source to the heat transfer medium and an inner tube. The inner tube comprises an inlet portion at a first end for receiving a solution of ammonia sodium thiocyanate, an outlet portion at an other end for recovering regenerated ammonia vapour and separating the regenerated ammonia vapour from the concentrated ammonia sodium thiocyanate solution, and a heat transfer portion arranged between the inlet portion and the outlet portion, the heat transfer portion located within the inner cavity of the outer shell. The ammonia thiocyanate solution absorbs heat from the heat transfer medium within the heat transfer portion of the inner tube and regenerates the ammonia vapour.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure will be made with reference to the drawings in which:

FIG. 1 depicts in a block diagram, a prior art refrigeration system;

FIG. 2 depicts in a block diagram, components of an efficient refrigeration system in accordance with the present disclosure;

FIG. 3 a depicts in a block diagram, components of a regenerator 215 in accordance with the present disclosure;

FIG. 3 b depicts in a block diagram, components of another regenerator 215 b in accordance with the present disclosure;

FIG. 4 depicts in a block diagram, components of an absorber 210 in accordance with the present disclosure;

FIG. 5 depicts in a schematic a refrigeration system using the regenerator and absorber in accordance with the present disclosure;

FIG. 6 depicts in a schematic a refrigeration system using the regenerator and absorber in accordance with the present disclosure;

FIG. 7 depicts in a schematic a refrigeration system using the regenerator and absorber in accordance with the present disclosure;

FIG. 8 depicts in a schematic a refrigeration system using the regenerator and absorber in accordance with the present disclosure; and

FIG. 9 depicts in a schematic a refrigeration system using the regenerator and absorber in accordance with the present disclosure.

DETAILED DESCRIPTION

An efficient refrigeration system is described that uses both absorption and adsorption techniques. The improved efficiency allows the refrigeration process to be driven by a low grade heat supply. The low grade heat supply could be, for example, exhaust heat from an engine, waste heat from an industrial process, solar energy, a biomass heat source, or a geothermal heat source. Other low grade heat sources may be used.

The efficient refrigeration system is described herein as using ammonia (NH3) and sodium thiocyanate (NaSCN) salt. The refrigeration process involves vaporous ammonia, sorbent liquid ammonia sodium thiocyanate solution, and solid sodium thiocyanate salts. The mixed solid (NaSCN) and liquid (NH3-NaSCN solution) sorbent provides strong sorptive ability for the vaporous ammonia. The sorptive mixture provides adsorption onto the solid surface of the NaSCN salts, and absorption into the NH3-NaSCN liquid solution. The two different sorption mechanisms are functioning with different magnitudes at different stages of the refrigeration process. Normally, a higher salt concentration can be chosen for higher heat source temperatures. However, the solid salts should be contained in the absorber only.

FIG. 2 depicts in a block diagram, components of an efficient refrigeration system in accordance with the present disclosure. The system 200 comprises an evaporator 205 coupled to an absorber 210. The evaporator 205 has low pressure refrigerant that absorbs heat from the cooling environment 202. The cooling environment 202 is the environment that is desired to be cooled. The heat may be transferred from the cooling environment 202 through a heat exchanger 203. The heat is transferred to the vaporous ammonia, which then passes to the absorber 210.

The absorber 210, absorbs the vaporous ammonia into the ammonia-sodium thiocyanate solution, and adsorbs the vaporous ammonia on to the surface of sodium thiocyanate salts. It is understood that although referred to as the absorber 210, it may both absorb and adsorb the vaporous ammonia. The sorbent mixture (ammonia sodium thiocyanate solution, and sodium thiocyanate salt) in the absorber 210 absorbs heat from the vaporous ammonia. This heat is dissipated from the system to the external environment. The absorber may dissipate the heat via, for example, liquid or air cooling, or the solution may be cooled after passing through the regenerator. The refrigerant-sorbent solution is pumped to the regenerator 215 using the pump 212. The sodium thiocyanate salt is not pumped to the regenerator but remains in the absorber 210.

The regenerator 215 heats the refrigerant-sorbent solution using a low grade heat source 217. The heating of the solution regenerates the ammonia vapour which is then passed to the condenser 220. as a result of vaporous ammonia being separated from the solution, the remaining ammonia-sodium thiocyanate solution is concentrated. This strong solution is then supplied to the absorber 210, where it will absorb/adsorb the vaporous ammonia. The strong solution may be cooled prior to being returned to the absorber. The strong solution may pass through a heat exchanger 218 to cool the solution and an expander valve 219 before entering the absorber.

The vaporous ammonia is supplied to the condenser 220 from the regenerator 215. The condenser 220 condenses the vaporous ammonia to liquid ammonia. The vaporous ammonia may be condensed by cooling the vapour using an air or liquid heat exchanger. The heat is dissipated to the external environment. The condensed liquid ammonia may be passed to an ammonia reservoir 222, then passed to the evaporator 205 through an expander valve 224, where the liquid evaporates and absorbs heat from the cooling environment.

In the above system 200, the evaporator 205 absorbs heat from the cooling environment 202. This heat is transferred to the absorber 210 via the refrigerant when the vaporous refrigerant is absorbed/adsorbed into the sorbent mixture. The vaporous refrigerant is absorbed/adsorbed by the sorbent and heat is generated. This heat is released to the external environment through a heat exchanger, cooling the sorbent mixture. The regenerator 215 heats the sorbent mixture using low grade heat. The heating of the sorbent mixture regenerates the refrigerant, causing the sorbent mixture to become more concentrated. The regenerated refrigerant is passed to the condenser 220, and the strong solution is passed to the absorber 210. The condenser condenses the refrigerant to liquid refrigerant. The liquid refrigerant can then be evaporated by the evaporator 205 to absorb heat from the cooling environment.

The regenerator 215 uses a low grade heat source to regenerate ammonia from the ammonia-sodium thiocyanate solution. When the ammonia is regenerated, the ammonia sodium thiocyanate solution becomes more concentrated and sodium thiocyanate salt may crystallize on surfaces of the regenerator 215. This crystallization can hinder the transfer of heat to the mixture from the low grade heat source, and so hinder the regeneration of the refrigerant. The regenerator may be designed to reduce the crystallization. The reduction of the crystallization increases the efficiency of the regenerator, and reduces the heat required from the low grade heat source.

FIG. 3 a depicts in a block diagram, components of a regenerator 215 in accordance with the present disclosure. The regenerator 215 uses a tube and shell construction. A single tube 305 is depicted in FIG. 3 a, however it is understood that multiple tubes may be present within the shell 310, depending on the required cooling capacity of the system. The shell 310 may be filled with a heat transfer medium 312 for transferring heat from the low grade heat source to the sorbent solution 314. The tube 305 has a regenerator tube inlet portion 320. The sorbent solution 314 enters the tube 305 of the regenerator 215 at the regenerator tube inlet portion 320. Once the solution 314 enters the tube 305, heat is transferred to the solution 314 from the low grade heat source by the heat transfer medium 312. The heat causes ammonia vapour 330 to form within the tube 305. The tube 305 is oriented vertically, causing the generated ammonia vapour 330 to rise, forming a vapour-liquid two-phase flow within the tube 305. The rising ammonia vapour circulating two-phase flow 330 helps to hinder crystallization of the concentrated solution 332 on the walls of the tube 305. At a solution outlet tube 322, the concentrated solution 332 is separated from the regenerated ammonia vapour 330 and returned to the absorber. The regenerated ammonia vapour 330 rises within the tube and is collected at an inner tube vapour outlet 335. The inner tube vapour outlet 335 is coupled to the condenser. The regenerated vapour passes from the inner tube vapour outlet 335 to the condenser. The solution outlet tube 322 may be located at various places along the inner tube 305. As shown in FIG. 3 a the solution outlet tube 322 is positioned a portion of the way up the inner tube 305. The solution outlet tube 322 may be located anywhere on the inner tube 305 below the height of the solution in the tube. The height of the tube provides enough distance between the top level of the solution in the tube and the top of the tube so that solution will not pass through the vapour outlet tube 335. The solution outlet tube 322 may be placed at the bottom of the inner tube 305, in which case the solution inlet portion may be located a portion of the way along the inner tube 305.

FIG. 3 b depicts in a block diagram, components of another regenerator 215 b in accordance with the present disclosure. The regenerator 215 b is similar in construction to that of FIG. 3 a, however the mixture 314 enters the shell 310 instead of the tube 305. The heat transfer medium 312 is in the tube 305. In this configuration, crystallization may occur on the exterior wall of the tube 305 and the interior wall of the shell 310, and the ammonia vapour formation may not be sufficient to prevent the formation. In this case a cleaning cycle may be used to periodically flush the crystals from the regenerator 215 b as described further herein.

The regenerator 215 (and 215 b) require input of ammonia-sodium thiocyanate mixture and the input of low grade heat. The low grade heat is transferred to the mixture and causes ammonia dissolved in the mixture to boil and form vapour, concentrating the ammonia-sodium thiocyanate. The regenerator provides the generated vapour to the condenser and the concentrated ammonia-sodium thiocyanate to the absorber.

FIG. 4 depicts in a block diagram, components of an absorber 210 in accordance with the present disclosure. The absorber 210 is constructed as a tube and shell. Ammonia vapour is supplied into the tube 405 from the evaporator 205 via a vapour inlet tube 417. The ammonia vapour forms ammonia bubbles 415 within the sorbent solution 420. The ammonia bubbles 415 cause the sorbent mixture to circulate within the tube and shell structure of the absorber. The absorber may also have anti fouling particles 425. The anti fouling particles 425 have the same density as or are slightly denser than the ammonia-sodium thiocyanate solution so that they sink to the bottom of the tube and shell absorber. The rising ammonia bubbles cause the anti fouling particles to rise with in the tube. The rising ammonia bubbles cause the sorbent mixture to circulate within the absorber. The sorbent mixture absorbs and adsorbs the ammonia bubbles causing the temperature of the mixture to rise. The solution is pumped from a solution outlet tube 410 of the absorber to the regenerator. Concentrated solution is returned to the absorber through a solution inlet tube 412 from the regenerator. The solution may pass through a heat exchanger to dissipate the absorbed heat. The heat exchanger may be for example cooling pipes 430 that circulate a cooling medium with the external environment.

FIG. 5 depicts in a schematic a refrigeration system using a regenerator and an absorber in accordance with the present disclosure. The system is substantially similar in construction and operation as refrigeration system 200 described with reference to FIG. 2. However, the refrigeration system 500 further comprises recovery expanders 505, 507, and 509. The recovery expander 505 is coupled between the condenser and the evaporator. The recovery expander 505 recovers energy from the expansion of the liquid ammonia to vaporous ammonia as the ammonia passes from the condenser to the evaporator. The energy recovered by the recovery expander 505 may be used to drive the pump of the system, reducing the required energy input required and increasing the efficiency of the system. The energy recovered from the liquid ammonia expansion process may be sufficient to supply a large portion of the energy required to drive the pump. By controlling the temperature of the liquid ammonia through the condenser, it may be possible to adjust the amount of energy recovered by the recovery expander to drive the pump. The higher temperature of the liquid ammonia, the more power might be produced, however, the less refrigeration capacity. Additional energy may be recovered from the expansion of the concentrated strong solution flowing from the regenerator to the absorber through the energy expander 509. A further recovery expander 507 is coupled between the connection between the regenerator and the condenser and the connection between the evaporator and the absorber. The recovery expander recovers energy from the pressure of the vapour. The refrigeration system 500 may utilize one or more of the recovery expanders to drive the pump of the system. The recovery expanders may provide a portion of the energy required to drive the pump.

FIG. 6 depicts in a schematic a refrigeration system using the regenerator and absorber in accordance with the present disclosure. The refrigeration system is similar to refrigeration system 500, however, the evaporator is coupled to the absorber through a heat exchanger 605. The heat exchanger 605 may cool the ammonia vapour prior to passing to the absorber. A further heat exchanger 607 may be coupled between the absorber and the regenerator. The warmed solution from the regenerator may be used to raise the temperature of the solution prior to entering the regenerator. The heat exchangers 605, 607 may be referred to as recuperators since the heat is exchanged within the refrigeration system and not with the external environment.

FIG. 7 depicts in a schematic a refrigeration system using the regenerator and absorber in accordance with the present disclosure. The refrigeration system is similar to refrigeration system 500, however, a compressor 705 is coupled between the evaporator and the absorber. The compressor 705 may be used to provide additional cooling capacity to the refrigeration system. The recovery expanders may be used to provide a portion of the energy necessary to drive the compressor 705.

FIG. 8 depicts in a schematic a refrigeration system using the regenerator and absorber in accordance with the present disclosure. The refrigeration system is similar to refrigeration system 500, however, a heat exchanger is coupled between the evaporator and the absorber. The heat exchanger cools the ammonia vapour using ambient cooling, such as air or liquid cooling.

FIG. 9 depicts in a schematic a refrigeration system using the regenerator and absorber in accordance with the present disclosure. The refrigeration system 900 is similar to refrigeration system 500, however, an additional energy expander 905 is coupled between the connection between the regenerator and the condenser and the connection between the evaporator and the absorber. The energy recovered from the pressure difference may be used to partially power the solution pump.

The refrigeration systems described above may further include a dryer to remove moisture brought into the system. A drying agent, such as calcium oxide, may be placed as a thin layer between of two pieces of fabric cloth to form a blanket form, which may then be rolled to form a cylinder. The cylindrical container may then be placed in the ammonia gas stream at the regenerator outlet to remove any moisture circulating within the system.

The refrigeration systems described above may include a flushing system to clean the refrigeration systems when the system shuts down. The flushing system sends warm ammonia liquid that is in the compressor through the pipes and couplings of the refrigeration system. The flushing system dissolves any crystallization of the salt that may be present in the system.

The salt used in the system, for example the sodium thiocyanate salts, should have a low water concentration, for example in the range of 0.2-0.3% by weight. The salt concentration of the sodium thiocyanate in the ammonia solution before charging into the system may be for example in the range of 40-60% by weight, or more preferably 47-53% by weight. Once the initial salt concentration of the solution is established, ammonia gas is introduced into the system to the design point. For example ammonia gas is added to the system until the salt concentration of the solution is approximately 50% by weight. The mixture may include a drying agent to remove any water present in the mixture. The mixture may include for example 0-6% by weight of drying agent to immobilize any water present in the system, or more preferably 0.01-4% by weight. The mixture may also have anti-corrosion additives. It is understood that the salt concentration of the mixture changes at different points within the system. For example the concentration of salt at the regenerator inlet is generally within the range of 40-71% by weight, while the salt concentration of in the mixture is generally within the range of 38-65% by weight.

The dimensions of the various components may be varied depending on the required cooling capacity of the system. The inner tubes of the regenerator typically have a diameter in the range of 0.75 to 1 inch (19 mm-25 mm). The number of inner tubes can be increased in order to increase the refrigeration capacity of the system. The length of the tubes may be determined based on the amount of solution present in the system and the amount of heat delivered from the low grade heat source.

One or more illustrative embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims. 

1. A refrigeration system for cooling a cooling environment using a low grade heat source, the refrigeration system comprising: a condenser for condensing vaporous ammonia to liquid ammonia using a first heat exchanger; an evaporator coupled to the condenser through a first valve, the evaporator for evaporating liquid ammonia to vaporous ammonia by absorbing heat from the cooling environment, the evaporator receiving the liquid ammonia from condenser; an absorber coupled to the evaporator, the absorber for absorbing vaporous ammonia into an ammonia sodium thiocyanate solution and for adsorbing vaporous ammonia onto sodium thiocyanate salts, the absorber receiving the vaporous ammonia from the evaporator; a regenerator for regenerating ammonia vapour from the ammonia sodium thiocyanate solution with the absorbed vaporous ammonia, the regenerator coupled to the condenser for supplying the regenerated ammonia vapour to the condenser; and the regenerator further coupled to the absorber for returning the concentrated thiocyanate solution to the absorber; and a solution pump for pumping the ammonia sodium thiocyanate solution with dissolved ammonia from the absorber to the regenerator.
 2. The refrigeration system as claimed in claim 1, wherein the regenerator is coupled to the absorber through a second valve and a second heat exchanger, the second heat exchanger cooling the ammonia sodium thiocyanate solution
 3. The refrigeration system as claimed in claim 2, wherein the first and second heat exchanger dissipate the heat with a cooling medium.
 4. The refrigeration system as claimed in claim 3, wherein the cooling medium is water, anti-freezing agent or air.
 5. The refrigeration system as claimed in claim 1, wherein the first valve comprises a first energy recuperator for recovering energy from the expansion of the liquid ammonia, the recovered energy providing energy to the solution pump.
 6. The refrigeration system as claimed in claim 2, wherein the second valve comprises a second energy recuperator for recovering energy from the flow of the concentrated ammonia sodium thiocyanate solution, the recovered energy providing additional energy to power the solution pump.
 7. The refrigeration system as claimed in claim 1, further comprising a compressor coupled between the expander and the absorber for compressing the vaporous ammonia.
 8. The refrigeration system as claimed in claim 1, further comprising a third heat exchanger coupled between the expander and the absorber for dissipating heat from the vaporous ammonia.
 9. An absorber for use in a refrigeration system comprising: an outer shell comprising: an outlet for removing ammonia sodium thiocyanate solution; an inlet for adding concentrated ammonia sodium thiocyanate solution to the absorber; and an inner cavity for holding: ammonia sodium thiocyanate solution for absorbing ammonia vapour, and sodium thiocyanate salts for adsorbing ammonia vapour; an inner tube having an inlet and outlet end, the inner tube located within the inner cavity of the outer shell; and a gas duct for transporting ammonia vapour into the ammonia sodium thiocyanate solution within the inner tube from the inlet end to the outlet end.
 10. The absorber as claimed in claim 9, wherein the inner cavity further holds anti-fouling particles that are of a similar density of the ammonia sodium thiocyanate solution.
 11. The absorber as claimed in claim 9, wherein the gas duct comprises a gas nozzle for bubbling the ammonia vapour into the ammonia sodium thiocyanate solution.
 12. The absorber as claimed in claim 9, wherein the outer shell is made of one of aluminium; steel; stainless steel; and titanium.
 13. A regenerator for regenerating ammonia vapour from a solution of ammonia sodium thiocyanate for use in a refrigeration system using a low grade heat source, the regenerator comprising: an outer shell having an inner cavity and a circulation means for circulating a heat transfer medium from the inner cavity to a low grade heat exchanger for exchanging heat from the low grade heat source to the heat transfer medium; an inner tube having: an inlet portion at a first end for receiving a solution of ammonia sodium thiocyanate; an outlet portion at an other end for recovering regenerated ammonia vapour and separating the regenerated ammonia vapour from the concentrated ammonia sodium thiocyanate solution, and a heat transfer portion arranged between the inlet portion and the outlet portion, the heat transfer portion located within the inner cavity of the outer shell; and wherein the ammonia thiocyanate solution absorbs heat from the heat transfer medium within the heat transfer portion of the inner tube and regenerates the ammonia vapour.
 14. The regenerator as claimed in claim 13, further comprising a plurality of inner tubes located partially within the inner cavity of the outer shell.
 15. The regenerator as claimed in claim 14, wherein longitudinal axes of the inner tubes are arranged substantially parallel to a longitudinal axis of the outer shell.
 16. The regenerator as claimed in claim 15, wherein the inner tubes are made from one of: aluminium; steel; stainless steel; and titanium.
 17. The regenerator as claimed in claim 16, wherein the inner tubes have a diameter in the range of 0.75 to 1 inch
 18. The regenerator as claimed in claim 17, wherein the low grade heat source is one of: a solar heat source; process waste heat; engine exhaust heat; generator exhaust heat; a biomass heat source; and a geothermal heat source. 